Field of the Invention
The invention relates to a receiving device for receiving video and teletext signals.
Such receiving devices are in the mean time commercially available television sets and video recorders in which teletext decoders are integrated. More and more broadcasting corporations are going over to transmitting not only television broadcasts but also additional information for the viewer in the form of teletext pages. In the case of World System Teletext, called teletext for short, the teletext data for the teletext pages to be displayed on the screen can be transmitted in the television lines 6 to 23 and 318 to 353 of the vertical blanking interval of the video signal, assuming a 625 line television system. The teletext data are transmitted continually in a specially encoded format at a bit rate of 6.9375 MHz and 360 bits per teletext line. Each teletext character that is transmitted requires a byte having 7 data bits and one parity bit. In Level 1 of World System Teletext, a teletext page consists of 24 teletext lines which each begin with teletext address data with subsequent teletext data.
The first teletext line of a teletext page has, as teletext address data, a magazine number with subsequent line number, page number, page sub-number and check bits, adjoined by 32 bytes of teletext data. By contrast, the teletext lines 1 to 23 are provided only with the magazine and line numbers as teletext addresses. The page numbers may lie between 100 and 899, the first digit identifying the eight magazines present in the teletext system.
If consecutive information requires more than one teletext page, subpages can be communicated. These subpages are likewise identified in the first teletext line of the teletext page.
The teletext pages transmitted by the broadcasting corporations are transmitted cyclically, that is to say that after all the pages contained in the editorial office of the broadcasting corporation have been transmitted, a cycle is ended. The same pages are retransmitted in the next cycle. In order that the cycle time and, consequently, the waiting time at the receiving end for the user of the teletext service remain reasonable, the number of teletext pages contained in a cycle is limited. The cycle time is about 20 seconds. The number of pages per cycle thus results from the cycle time and the number of teletext data lines used per vertical blanking interval.
In principle, the teletext data are concomitantly transmitted together with the video signal data in the abovementioned time frame, which means that the teletext data are transmitted in specific lines which are indiscernible on the screen. In this case, the teletext data are present at the receiver input in analog form, just like the video signals. In order to be able to decode the teletext data, the latter are regularly separated from the video input signal by a suitable circuit configuration. This is done by a so-called data slicer. In this case, the video input signal, generally the CVBS signal, is clamped by means of a capacitor and sampled by means of a sample & hold circuit, and then digitized in an analog/digital converter. The now digitized teletext signal is fed to a control device or a control block for further processing. A sync information item can be recovered from the digitized data. This sync information is used to establish exactly the time slots for which the teletext data to be decoded are transmitted.
Furthermore, in such receiving devices, a multiplicity of analog signals have to be digitized so that they can be processed further in a microcontroller of the receiving device. One example of such analog signals is the keypad interrogation of a remote control. In that case, by actuating keys, an operator alters for example the volume or the brightness of the picture to be displayed on a screen. This signal is initially present at the remote control receiver in the form of a modulated analog signal, which must be digitized for further processing in the microcontroller.